Design and Characterization of a Pin-to-Water Plasma Reactor Setup for the Generation of High Strength Plasma Activated Water

Abstract

Plasma-activated water (PAW), generated by non-thermal atmospheric plasma interaction with water, has emerged as a promising solution due to its potent antimicrobial, antitumor, and nitrogen fixation properties, with applications in agriculture, healthcare, and food industry. The plasma-water interaction produces reactive oxygen and nitrogen species (RONS) in water. Long-lived species like hydrogen peroxide (H2O2), nitrite (NO2-), and nitrate (NO3- are formed in PAW. However, tailoring PAW chemistry to specific applications remains challenging due to variations in plasma reactors and generation methods. This study addresses these challenges by examining PAW chemistry and assessing its efficacy in multidrug-resistant pathogens. This study is structured around three main objectives to enhance the understanding of PAW generation and its chemical reaction mechanism for generating stable PAW chemistry for antibacterial application. The first objective was to identify and quantify the critical factors in an air discharge pin-to-water (P2W) reactor setup to generate high-strength PAW (hs-PAW) for antibacterial application. Critical factors of a P2W reactor setup, including initial water chemistry, external water cooling, enclosure around the discharge, and activation time, were studied. The resulting hs-PAW contained 650 mg/l of NO2- and 215 mg/l of H2O2, while maintaining a neutral pH range. These species remained stable for 15 days, enhancing hs-PAW's shelf life and utility. Further exploration revealed the impact of operating current on nitrogen oxides (NOX) generation in air phase in the P2W setup and its effect on the PAW chemistry. High-current mode operation of the P2W reactor (HCM, 32.3 mA) significantly increased H2O2 and NO2- concentrations in PAW to 161 mg/l and 1070 mg/l, respectively, compared to low-current mode (LCM, 19.5 mA). The second objective was to understand the fluid circulation in the P2W setup. The cold-flow CFD simulations using Ansys Fluent® revealed that enclosing the reactor enhances fluid recirculation, thus increasing the NOX availability at the air-water interface and reducing gas species loss. Chemical kinetics modeling using Chemical Workbench® (CWB) provided further insights. Bulk-phase air discharge simulations incorporating air-water interface reactions confirmed that approximately 90% of PAW chemistry was driven by interface reactions, with NO2- formation in water directly proportional to NOX production in the gas phase. The third objective evaluated the antibacterial efficacy of hs-PAW, which was tested against multidrug-resistant pathogens, including MRSA and hvKP. The hs-PAW retained its biocompatibility and antimicrobial potency for 15 days as a therapeutic solution and demonstrated its potential as a stable surface disinfectant. This work advances the understanding of hs-PAW generation and characterization. The insights gained here pave the way for scaling hs-PAW production using P2W setup for larger volumes, with broad applications in healthcare and beyond.